Method for electrolysis



Nov. 30, 1965 s. G. osBoRNE METHOD FOR ELECTROLYSIS 2 Sheets-Sheet 1Filed Aug. 3, 1960 Naacog BRINE SOLUTION OUT Nov. 30, 1965 s. G. osBoRNE3,220,941

METHOD FOR ELECTROLYS IS Filed Aug. .'3, 1960 2 Sheets-Sheet 2 METHD FRELECTROLYSS Sidney G. sborne, deceased, late of St. Davids, Ontario,Canada, by Canada Trust Company, administrator, St. Catharines, ntario,Canada, and George T., li/iiier, Lewiston, NX., assignors to HookerChemical Corporation, Niagara Fails, NX., a corporation of New YorkFiled Aug. 3, 1960, Ser. No. $7,331

13 Ciaims. (Cl. 2MP-87) This application is a continuation-impart of ourcpending application Serial No. 267,846, led January 23, 1952, now U.S.2,967,807, and of our co-pending application Serial No. 327,182, tiledDecember 22, 1952, now abandoned.

This invention relates to improvements in the electrolytic decompositionof chemical compounds and the production of useful products therefrom.Further, this invention relates to novel methods for .advantageouslyemploying the loss in current etiiciency inherent in the use ofpermselective diaphragms in electrolytic cells, caused by undesirableleakage, migration or adsorption of a useful product of electrolysisinto or through the diaphragm, under the intluence `of a concentrationgradient. More particularly, this invention involves a method whichresults in the production of an additional product of the cell in anadded compartment of a multicompartment electrolytic cell, whichcomprises reacting a chemical reagent With the liquid or solubleproducts appearing in the additional compartment as a result ofundesirable leakage, migration or adsorption into or through thepermselective diaphragm. Still further, this invention relates to anovel structure of permselective diaphragms for use in the electrolyticdecomposition of chemical compounds which avoids the loss of currenteiciency .and provides for separate recovery of another product of thecell in which it is used. Still more particularly this invention relatesto methods and apparatus for the production and separate recovery ofhydrogen, chlorine, caustic soda and soda ash by the electrolyticdecomposition of an aqueous solution of sodium chloride in anelectrolytic cell separated into three compartments by permselectivediaphragms.

Permselective diaphragms are characterized by their substantialimpermeability to liquids and by their selective permeability to ions ofone charge and substantial impermeability to ions of the opposite chargewhen wet with an electrolyte and under the infiuence of an electricalcurrent. Permse'lective diaphragms which selectively permit the passageof `anions are designated as anionic; those which selectively permit thepassage of cations are designated as cationic. The practical utilizationof permselective diaphragms in electrolytic cells for the electrolyticdecomposition yof chemical compounds, and the separate recovery of theproducts resulting therefrom, depend upon the presence of a polarmedium, generally Water, being present in the pores of the diaphragm;that is to say, permselective diaphragms must be Wet with solvent inorder to function in accordance with this invention. Such diaphragms andcertain methods and apparatus for employing them in the electrolyticdecomposition of chemical compounds are more fully set forth in ourco-pending application for Letters Patent S.N. 267,- 846 tiled January23, 1952.

In our co-pending application S.N. 267,846 there is dis- States Patentfiii Patented Nov. 3Q, 1965 ICC closed the fundamental concept ofsimultaneously controlling both molecular yand ionic migration duringthe electrolysis of chemical compounds between the electrodes of anelectrolytic cell, by electrolyzing the chemical compound in a cellseparated into electrode compartments, by one or more permselectivediaphragms. By this means, and under ideal conditions, includingsubstantially one hundred percent permselectivity of the permselectivediaphragms employed and complete impermeability to solvents,substantially all of the electrical current introduced duringelectrolysis will be carried through the permselective diaphragm by ionsof one charge, because permselective diaphragms do not permit thepassage of the ions of opposite charge. However, neither one hundredpercent permselectivity nor complete impermeability to solvents in suchdiaphragms has yet been achieved in practice.

It follows therefore, that in practice, even under the most perfectconditions, that there is a minor transfer of such polar medium orsolvent through the diaphragm and a minor transfer of undesirable ionsin the direction opposite to desired particularly high speed ions,because of the presence of such solvent; and that, this causes adecrease in ampere eiciency and a loss of other advantages. In addition,there is also a loss in the permselectivity of such .a diaphragm, withincreasing concentration of the products of electrolysis contained inone or more of the electrode compartments. The decreasingpermselectivity with increasing caustic concentration appears to followthe curve of typical adsorption isotherms. Thus, when permselectivediaphragms .are used in electrolytic cells, the current efficiencydecreases with increasing concentration of the pro-duct in its electrodecompartment of the cell, corresponding to the proportional loss ofpermselectivity of the diaphragm. Although we do not Want to be held tothis explanation, our invention may 'be more easily understood byillustrating the case of producing caustic soda in terms of it.

For example, when sodium hydroxide and elemental chlorine are producedby the electrolytic decomposition of an aqueous solution of sodiumchloride brine in an electrolytic cell in which the brine at the anodeis separated from the caustic soda produced at the cathode, by acationic permselective diaphragm, the diaphragm operates in two distinctmanners, the rst advantageously and the second adversely.

First, during electrolysis sodium ions migrate from the anolyte to thecathode, and upon reaching the cationic permselective diaphragm, whichdivides the cell into separate anode an-d cathode compartments, arepermitted passage through the diaphragm from active point to activepoint in the pores of the cationic permselective diaphragm because suchdiaphragm permits passage of such sodium cations, thus permitting theisolation and separate recovery yof sodium hydroxide in the cathodecompartment and also permitting an increase in its concentration withcontinued electrolysis. But the second mode oisets this advantagebecause, under the influence of the concentration gradient, sodiumhydroxide diffuses into the diaphragm from the cathode compartment inthe direction toward the anode by adsorption because of the presence ofthe polar solvent in the pores of the diaphragm, thus causing a seriousloss in current eiciency and other disadvantages. The effects of thissecon-d described mode of operation of permselective diaphragms aresuicient to negate some of the important advantages resulting from thefirst describe-d method of operation for the permselective diaphragm.

This is vfurther illustrated, for example, by employing a presentlyavailable ca-tionic permselective diaphragm in a two compartmentelectrolytic cell for the electrolysis of an aqueous solution of sodiumchloride to hydrogen, caustic soda and chlorine and observing thecurrent etiiciencies at various concentrations of caustic in thecatholyte. The current efficiency of such cell is found to decrease from75 percent at a caustic concentration in the catholyte of ten gramssodium hydroxide per liter to 46 percent current efiiciency as thecaustic concentration is increased in the catholyte to 500 grams sodiumhydroxide per liter. The following tabulation gives these and othercurrent efficiencies which We have experimental-ly determined at various-caustic concentrations:

Caustic concentration, g.p.l.: Current eiciency, percengt These dataconstitute a typical adsorption isotherm and at concentrations ofcaustic in the catholyte of 500 grams sodium hydroxide per liter andabove, the current elliciency remains at 46 percent. This indicates thatthe percent permselectivity of the diaphragm has been reduced to zeropercent and that the advantages to be realized by using thepermselective diaphragms are offset by the deleterious effects ofadsorption of caustic into the permselective diaphragm. Under theseconditions of high caustic concentration in the catholyte, the diaphragmapparently operates simply as a porous diaphragm in the manner that anasbestos diaphragm will operate to show a current eiciency of about 46percent irrespective of the concentration of caustic in the catholyte.Thus in the case of using a permselective diaphragm for making highconcentration caustic or in the case of using an asbestos diaphragm formaking caustic soda of any strength, ap-

proximately half the current input is being carried by ions of onecharge while approximately the other half is carried by ions of theopposite charge.

In our co-pending application, S.N. 267,846, we have proposed toincrease the percent permselectivity of the permselective diaphragmat agiven caustic strength, that is to say, to offset the elfects of theundesirable adsorption of sodium hydroxide into the cationic diaphragmfrom the cathode compartment in the direction toward the anodecompartment, by using two diaphragms in the electrolytic cell therebyforming a center compartment between them, into which water isintroduced whereby the concentration of the caustic soda in contact withthe diaphragm facing the anode, and subject to adsorption by suchdiaphragm, is very dilute in comparison to the caustic soda beingproduced in the cathode compartment, whereby the loss in permselectivityof the diaphragm facing the anode is correspondingly reduced and currentefciency correspondingly increased, then transferring the weak causticsoda solution so produced in the center compartment to the cathodecompartment, by way of a conduit in communication between the center andcathode compartments. Another technique for offsetting the ineiciency ofpermselective diaphragms, which is disclosed in our co-pendingapplication for patent, involves employing a porous diaphragm facing thecathode and permitting a slow percolation of electrolyte through it inthe' direction toward the cathode thereby protecting the cationicpermselective diaphragm which faces the anode from the strong caustic inthe cathode compartment. Still another technique for offsetting theineliciency of the permselective diaphragms, disclosed in our co-pendingapplication above referred to, based on our finding that the diffusioncoefficient for sodium carbonate through a permselective diaphragm is ofthe order of about one tenth of the diffusion coefficient for sodiumhydroxide through such diaphragms under the same conditions, involvespassing carbon dioxide into the catholyte in the cathode compartment ofthe cell, or outside the cell, to cause the formation and separation ofsodium carbonate rather than caustic soda.

We have now found that the losses in current efficiency may be minimizedand an additional product of the cell may be realized by employing apermselective diaphragm structure of this invention in the electrolysisof chemical compounds in an electrolytic cell, said structure beingcomposed of two diaphragms having the same kind of permselectivityseparated from each other to form a center compartment containing aninlet for introducing a chemical reactive with the products ofelectrolysis migrating from the electrode compartments into saidcompartment and an outlet for removing the products so produced.

We have now also found that we can increase the percent permselectivityof permselective diaphragms in processes and apparatus for theelectrolysis of alkali metal chloride brines by approximately the amountof permselectivity lost due to adsorption and with the realization of afourth product of the cell, alkali metal carbonate, by effecting theelectrolysis of an aqueous alkali metal chloride solution in anelectrolytic cell divided into an anode compartment containing brine, acenter compartment, and a cathode compartment, by two cationicpermselective diaphragms, carbonating the solution in the centercornpartment with carbon dioxide and separately recovering the alkalimetal carbonate so produced and alkali metal hydroxide produced in thecathode compartment, in addition to separately recovering the chlorineand hydrogen which are also produced.

We have also found that We can eifectively regulate the ratio of alkalimetal carbonate to alkali metal hydroxide produced in accordance withour invention, by controlling the concentration of caustic soda producdin the catholyte. This finding is of vast commercial signiiicancebecause it allows for maintaining an economic balance in the productionof these two basic commodities of commerce, according to the needs ofcommerce.

We have also found that the advantages of employing permselectivediaphragms in electrolytic cells for the electrolytic decomposition ofchemical compounds may be realized, without the concomitant undesirableeffects resulting from the adsorption and diffusion of liquid or solubleproducts of electrolysis from one electrode compartment toward theopposite electrode, and with the production of a separately recoverableadded product of the cell, by effecting the electrolysis of the chemicalcompound in an electrolytic cell divided into an anode compartment, anadded compartment and a cathode compartment, by two permselectivediaphragms, introducing a chemical reagent reactive with the liquid orsoluble products of electrolysis produced in the electrode compartments,into the solution ofthe center compartment and separately recovering theadded product of the cell so produced and the catholyte so produced, inaddition to separately recovering any gaseous products which are alsoseparately produced at the electrodes.

In order that this invention may be more readily understood it isdescribed with specic reference to certain preferred embodiments thereofand with reference to FIG- URE I, which is a diagrammatic sketchillustrating the electrolysis of an aqueous solution of sodium chlorideat the anode, the carbonation of the solution in the center compartmentof our three compartment electrolytic cell with carbon dioxide, wherebyhydrogen, chlorine, sodium carbonate and caustic soda are separatelyproduced and recovered; and, with reference to FIGURE II which is aperspective view of a permselective diaphragm structure of thisinvention: however, our invention is not to be construed as limitedthereto, as will be apparent from the modifications discussedhereinafter, except as dened in the appended claims.

Referring to FIGURE I, which is a diagrammatic sketch illustratingspecific embodiments of this invention: the electrolytic cell comprisesa vessel 1 separated into an anode compartment 2, a center compartment 3and a cathode compartment 4 by two cationic permselective diaphragms 5and 6. The anode compartment contains an anode 7 in contact with sodiumchloride brine 8. The anode compartment also contains a brine inlet 9, abrine outlet 10 and chlorine outlet 11. The cathode compartment containsa cathode 12 in contact with water or caustic 13. The cathodecompartment also contains a water inlet 14, a caustic outlet 15 and ahydrogen outlet 16. The center compartment 3 is provided with inlets 17for introducing water 13 and carbon dioxide 19 into the centercompartment of the cell, which is positioned between the cationicpermselective diaphragm facing the anode and cationic permselectivediaphragm facing the cathode, designated herein as 5 and 6,respectively. The center compartment is also provided with an outlet 29for removing the sodium carbonate solution which is produced under theconditions maintained during electrolysis. In addition the electrolyticcell contains any other necessary accessories for the givenelectrolysis.

Referring to FIGURE II which is a perspective view of a permselectivediaphragm structure of this invention composed of two sheets ormembranes 5 and 6 having the same kind of permselectivity, i.e. both arecationic or both are anionic. The permselective diaphragms are separatedfrom each other by a gasket type separator 21 containing inlet 17 andoutlet 20. The separated diaphragms form compartment 3 which contains anelectrolyte comprising a chemical reactive with the products ofelectrolysis migrating into said compartment from the electrodecompartments during electrolysis, some of the unreacted migratoryproducts of electrolysis and the reaction product of these substanceswith each other.

Referring back to FIGURE I, in accordance with this invention asaturated solution of sodium chloride brine 8 is fed into the anodecompartment 2 of the cell 1 through brine inlet 9. Depleted brine isremoved from the anode compartment through brine outlet 10. Chlorineproduced is removed from the cell through outlet 11. A starting solutionof sodium carbonate is introduced into compartment 3 in order to lowerthe voltage at the start of electrolysis, then water 1S and carbondioxide 19 are fed into the center compartment of the cell through inlet17. The sodium carbonate solution produced in the center compartmentduring the electrolysis is removed from the cell outlet 20. The causticsoda produced in the cathode compartment 4 is removed through outlet 15.Hydrogen which is also produced in the cathode compartment is removedthrough outlet 16. Water can be introduced into the cathode compartmentthrough inlet 14 in predetermined amounts in order to produce causticsoda of the desired concentration. In an alternative mode of operationthe cathode compartment may acquire water through the diaphragm 6 byelectro-osmosis, especially if the particular diaphragms employed haveminor leakage points. In such case the amount of water introducedthrough inlet 14 will be correspondingly minimized or entirelyeliminated. Another alternative mode of operation involves introducingthe carbon dioxide necessary to cause the formation of sodium carbonate,which is the fourth product of the cell in accordance with thisinvention, into the solution produced in the center compartment of thecell, at a point removed from the electrolytic cell, such as at a pointsubsequent to outlet 20 and circulating the sodium carbonate solution soproduced back into the center compartment to favor maintaining asubstantially uniform concentration of sodium carbonate throughout thesolution and adding water as necessary to do this.

Our findings to the effect that the percent permselectivity ofpermselective diaphragms at a given caustic concentration can beincreased by approximately the amount of permselectivity lost due toadsorption, and, with the realization of an additional product of thecell are best exemplied by data taken from a curve based on actualelectrolysis of a saturated aqueous solution of sodium chloride,producing different concentrations of caustic in an electrolytic celldesigned and operated after the principles depicted in the attachedfigure, in accordance with this invention and the foregoing description.

(B asis N 2120) Caustic Soda Ash Current Current Caustic Concentra-Eiciency Efficiency Total Current tion, gpl. N aOH and Percent andPercent Eiciency Production Production as as Caustic Soda Ash PercentPercent Percent These figures show that the deleterious effect ofadsorption of a desired product into permselective diaphragms can betransformed into an advantage by producing another useful product andthat the proportions of the two products produced, which can beseparately recovered, can be varied as desired. More particularly thedata also show that percent current efficiency can be consistentlymaintained during electrolysis in the electrolytic cell regardless ofthe concentration of sodium hydroxide in the catholyte being producedand that the lost current efciency based on caustic duc to adsorption isadvantageously employed in making sodium carbonate. The proportion ofcaustic soda to soda ash which is produced under a given set ofconditions of caustic concentration in the catholyte in an electrolyticcell having a total overall current efciency of 95 percent is also givenby the data in the tabulation. For example, if it is desired to maintainapproximately even distribution between the two products, under theconditions obtained during the electrolysis depicted, this can berealized by maintaining the concentration of sodium hydroxide in thecatholyte at about 500 grams per liter.

It has long been desired to maintain an economic balance between theamount of chlorine and caustic soda produced electrolytically. However,because of the basic laws of electrochemistry, equivalent amounts ofthese commodities must necessarily be produced in the electrolysis ofsalt. The present invention provides economic and feasible means forelectrolyzing brine so that chlorine is produced with the production ofan equivalent amount of alkali in the form of caustic soda and soda ashin exible predetermined ratios, thereby allowing for an economic balancebetween these products as the demand varies.

The following examples are given to further illustrate this inventionand should not be construed as limiting except as defined in appendedclaims.

Example l A saturated solution of sodium chloride brine was introducedinto the anode compartment of a three compartment electrolytic celldesigned after the principles depicted in the attached drawings. Theanode compartment, which contained a graphite anode, was separated fromthe center compartment by a cationic permselective diaphragm and thecathode compartment, which contained a steel cathode, was separated fromthe center compartment by another cationic permselective membrane. Thebrine was continuously introduced into the cell and circulated withinthe anode compartment through a conduit in communication with the brineinlet and outlet. A solution of sodium carbonate containing about 138grams per liter Na2CO3 was introduced into the center compartment of theelectrolytic cell to lower the initial voltage of the cell. Thissolution was maintained in circulation in the center compartment througha conduit connecting the inlet and outlet. Carbon dioxide was introducedinto the solution circulating in the center compartment at a point inthe outside conduit which connected the center compartments inlet andoutlet so that the solution circulated under the power of the gas liftthereby created and so that the concentration of sodium carbonate wassubstantially evenly distributed throughout the solution both inside andoutside the cell. The cathode compartment was filled with water. Thecell was operated to produce sodium carbonate, sodium hydroxide,chlorine and hydrogen. A cell temperature of about 60 degrees centigradeat a current density of 90 amperes per square foot on the diaphragms anda voltage of 4.65 volts were observed. The current efficiency on thecaustic soda was measured by employing a copper coulometer and found tobe 73.8 percent of theory at a NaOH concentration of 65.6 grams perliter of sodium hydroxide in the catholyte, and, the current efficiencyon the sodium carbonate at a Na2CO3 concentration of 190 grams per literwas found to be 21 percent of theory. The over-all cell Currentefficiency measured 94.7 percent of theory.

Example 2 In another experiment for the electrolysis of saturatedsolution of sodium chloride brine, in a manner after the foregoingexample, the current eiciency on caustic soda was found to be 54.8percent of theory at a NaOH concentration of 326.4 grams per liter ofsodium hydroxide in the catholyte, and, the current efficiency on thesodium carbonate at a Na2CO3 concentration of 420 grams per liter wasfound to be 40.0 percent of theory. The overall cell current efciencymeasured 94.8 percent of theory.

The permselective diaphragms employed in the electrolytic decompositionsdescribed in the preceding examples can be constructed using ionexchange resins Which have been formed 4into continuous thin sheets.When a cation active ion exchange resin is employed a cationicpermselective membrane is produced, i.e., one Which selectively permitspassage of cations through its structure from one compartment of thecell to the next adjacent compartment in the direction toward theattraction of its electrode under the iniiuence of an impressed voltageand when wet with electrolyte. When an anion active ion exchange resinis employed an anionic permselective membrane is produced, i.e., onewhich selectively permits passage of lanions through its structure fromone compartment of the cell to the next adjacent compartment in thedirection toward the attraction of its electrode under the iniiuence ofan impressed voltage and when wet with electrolyte. Successfuldiaphragms for use in electrolysis of alkali chloride brines have beenconstructed using cation exchange resins of an Amberlite type which havebeen formed into continuous sheets. These diaphragms .are described inUnited States Patents 2,681,319 and 2,681,320. Such sheets arecontinuous, nonporous, self-supporting, pliable, permselective membranesor pellicles which comprise a matrix-such as a synthetic hydrocarbontype plastic, or vulcanized, natural or synthetic rubber or polyethyleneor polyisobutylene, or polyvinyl chloride, or copolymers of vinylchloride and vinyl esters of lower aliphatic acids-having distributedintimately and uniformly therein particles of an insoluble, infusibleion-exchange resin, said particles being of such :a size as to passthrough a United States Standard sieve No. 50 andbeing present in saiddiaphragm in an amount equal to twenty-live to seventy-five percent ofthe total weight of the diaphragm. The degree of perfection attainablein operation when using such diaphragms in electrolysis is among otherthings a function of its tightness, or the number of macropores orleakage points occurring between the resin particles in the sheet orfiber.

The permselective diaphragm structures of this invention may be made ina number of diiferent Ways, depending on various factors particularly ingeometric design of the cell itself. One method is to use two sheets ofmembrane at least one of which is made in accordance with United StatesPatent 2,681,319 or United` States Patent 2,681,320, separated by agasket-type spacer having the inlets and outlets to the resultingcompartment positioned either in the spacer or through the membranes.Another method involves forming a continuous envelope from the saidAmberlite type permselective diaphragm and allowing a space between thefolded sheets to form the special compartment. When the cell design issuch as to require large sheet-like permselective diaphragms to bespaced from each other by a gasket-type spacer We have found itdesirable to reinforce at least one of the diaphragms by placing a gridagainst the face of the diaphragm in order to prevent buckling. The gridmay consist of a rigid screen or 4similar type structure of suitablematerial of construction disposed across the entire surface of thediaphragm. If la diaphragm buckles inward toward the center compartmentof the permselective diaphragm structure of this invention due topressure from an adjacent compartment a grid is positioned in contactwith the face of the diaphragm in said center compartment between thetwo diaphragms and is adapted to prevent buckling and contact lbetweenthe diaphragms. If a diaphragm Ibuckles outward toward an adjacentcompartment due to pressure from the center compartment of thepermselective diaphragm structure of this invention, a grid ispositioned in the adjacent compartment, in contact with the face of thediaphragm and is adapted to prevent buckling and contact with theelectrode if the next adjacent compartment is an electrode compartment.

The scope of this invention is not to be construed as limited to thespecific electrolysis depicted in the above examples, but is to includethe electrolysis of all those compounds which give products of fastmoving ions which result in undesirable back-migration effects throughirnperfect permselective membranes. Among the preferred chemicalcompounds which can be electrolyzed in accordance with the method ofthis invention are solutions of soluble inorganic salts such as theaqueous solutions of the alkali and alkaline earth metal bromides,chlorides, sulfates, acid sultes, etc. The ,alkali metal salts such aslithium and potassium which are very soluble and electrolyzable in watercan be successfully electrolyzed in the same manner as the sodium saltsdepicted above. For example, in the electrolysis of sodium sulfatesolutions las described in our copending application S.N. 267,846 ledJanuary 23, 1952, the salt is introduced into the center compartment ofa three compartment electrolytic cell, separated from the cathodecompartment by a cationic permselective membrane and separated from theanode compartment by an anionic permselective membrane, if the hydroxylions produced in the catholyte have a tendency to back-migrate due to anineicient cationic permselective diaphragm, another cationicpermselective membrane may be added between the center and cathodecompartments forming, in effect, a fourth compartment whereby an acidicreagent such as carbon dioxide may be added to improve the over-allcurrent efficiency and obtain other important advantages las discussedhereinbefore. (Similarly, if the hydrogen ions have a tendency toback-migrate due to .an inefficient anionic permselect1ve diaphragm anadditional anionic permselective membrane may be added between the anodeand center compartment forming in effect an additional compartmentwhereby a basic reagent may be added to improve the over-al1 currentefficiency and obtain other important advantages as discussedhereinbefore.)

Thus, .a preferred embodiment of this invention comprises the use of apermselective diaphragm type structure in an electrolytic decompositioncell into which a reactive chemical selected from the group consistingof acidic and basic substances is introduced in the center compartmentof said structure, which chemical is reactive with ions selected fromthe group consisting of hydroxyl and hydrogen or hydronium ions whichleak, back-migrate, or are absorbed into said center compartment of thepermselective diaphragm structure.

Chemical reagents other than carbon dioxide may be used in accordancewith this invention for realizing the improvements described. Thereagent selected will depend upon the salt to be electrolyzed and theproduct which is desired. Inorganic and organic acids or substancescapable of reactint7 with bases such as SO2, HCl, H23, H5904, aceticacid, benzoic acid, etc. may all be used to react with the migratinghydroxyl ions coming from the cathode compartment through the imperfectcationic permselective membranes. Similarly where it is desired to add abase into an additional compartment to prevent fast moving hydrogenion-s from reducing the current etiiciency by being lost because ofleakage in the anionic permselective membrane, the choice of this basewill depend upon the salt to be electrolyzed and the product which isdesired, as above. Alkali metal hydroxides or carbonates are among thepreferred compounds to be used. In either case the product need not .bewater soluble, but this is desirable especially where the reagent isintroduced directly into the cell compartment. The metallic saltscorresponding to the acid or base introduced into the center compartmentof the diaphragm structure of this invention will of course be producedlby the reaction between the acid or base and the hydroxyl or hydrogenions. For example, when the reagent added is sulfur dioxide theadditional product produced by the cell will be sodium suliite; bycontrolling the proportion of chemical reactant introduced, sodiumbisulte may be produced.

The operating conditions such as current density, applied voltage,temperature, feed and product concentrations, various additives andother conditions familiar to those skilled in the art are considered tobe unrelated in so far as limiting the scope of this invention.

The purity of the products in accordance with this invention are ofexceptionally high standard and are in accordance with the disclosure ofour co-pending application referred to above. By operating anelectrolytic cell embraced within the scope of this invention,particularly as depicted in the foregoing examples and descr-iption withreference to the drawings, elemental chlorine having a purity of 99.0percent can be consistently obtained and substantially pure hydrogenwith no detectable impurities is also consistently obtained. The liquidproducts of the cell are likewise of exceptional purity. A typicalanalysis of these products is as follows:

Diaphragm Structure Center Com- Catholyte Solution partment Solution43.0 percent NaOH 18.0 percent N :12003 4.0 p.p.m. Fe (soin. basis) 0.8p.p.m. Fe (soln. basis) Thus, in accordance with this inventionsubstantially pure products of the cell can be separately recovered inhigh concentrations.

Although we have described our invention with respect to certainspecific embodiments thereof we do not intend to be limited theretoexcept as delined in the following claims.

We claim:

1. In the process for the electrolytic ldecomposition of an aqueoussolution of an inorganic salt in an electrolytic cell having an anodecompartment containing anode means separated from a cathode compartmentcontainino cathode means by a diaphragm unit structure comprising twoessentially continuous non-porous permselective diaphragm-s having t-hesame kind of permselectivity and spaced from each other to form acentral compartment of said unit structure, said central compartmenthaving an inlet and an outlet which are in external communication witheach other through a conduit, and at least one of said diaphragmscomprising (a) a matrix which is polymeric material from the classconsisting of polyethylene, polyisobutylene, vulcanized natural andsynthetic rubber, polyvinyl chloride and copolymers of vinyl chlorideand vinyl esters of lower aliphatic acids and (b) particles of aninsoluble -infusible ion-exchange resin intimately and uniformlydispersed throughout said matrix, said particles 'being of such size asto pass through a United States Standard sieve size No. 50 and beingpresent in said diaphragm in an amount equal to twenty-tive percent toseventy-tive percent of the total weigh-t of said diaphragm, theimprovement which comprises: introducing the `said salt solution into anelectrode compartment; effecting the electrolysis of said solution byimpressing a decomposition voltage across the electrodes of said cell,whereby a product of electrolysis migrates into said centralcompartment; introducing into said external conduit a chemical compoundreactive with said product of electrolysis to form a reaction productwhich under the conditions of electrolysis has a diusion coefficientthrough the diaphragm no greater than about the diffusion coefficient ofcarbonate ion through the diaphragm; and separately recovering theproducts of the cell so produced.

2. lhe process of claim 1 wherein the said reactive chemical compoundintroduced is one which when dissolved in water is a base, and theproduct of electrolysis is hydrogen ion.

3. The process of claim It wherein the said reactive chemical compoundis one which when dissolved in water 1s an acid, and the product ofelectrolysis is hydroxyl ion.

4. The process of claim 3 wherein the compound is carbon dioxide.

5. The process of claim 3 sulfur dioxide.

6. In a process for the electrolytic decomposition of an aqueoussolution of an alkali metal salt in an electrolytic cell having an anodecompartment containing anode means separated from a cathode compartmentcontaining cathode means by a diaphragm unit structure comprising twoessentially continuous non-porous cationic permselective diaphragms andspaced from each other to form a central compartment of said unitstructure, said central compartment having an inlet and an outlet whichare an external communication with each other through a conduit, and atleast the diaphragm facing the cathode compartment comprising (a) amatrix which is polymeric material from the class consisting ofpolyethylene, polyisobutylene, vulcanized natural and synthetic rubber,polyvinyl chloride and copolymers of vinyl chloride and vinyl esters oflower aliphatic acids and (b) particles of an insoluble infusible cationexchange resin in-timately and uniformly dispersed through said matrix,said particles being of such as size as to pass through a United StatesStandard sieve size No. 50 and being present in said diaphragm in anamount equal to twenty-five percent to seventy-tive percent of the totalweight of said diaphragm, the improvement which comprises: introducingthe said wherein the compound is salt Solution into the anodecompartment; effecting the electrolysis of said solution by impressing adecomposition voltage across the electrodes of said cell, wherebyhydroxyl ions migrate into said central compartment from said cathodecompartment; introducing into said external c-onduit a chemical compoundwhich when dissolved in water is an acid reactive with said hydroxylions -to form a reaction product which under the conditions ofelectrolysi-s has a diffusion coefficient through the diaphragm nogreater than about the diffusion coeicient of carbonate ion through thediaphragm; and separately recovering the products of the cell soproduced.

7. In a process for the electrolytic decomposition of an aqueoussolution of an alkali metal chloride in an electrolytic cell to producehydrogen, chlorine, alkali metal hydroxide and an additional product ofthe cell, having an anode compartment containing anode means separatedfrom a cathode compartment containing cathode means by a diaphragm unitstructure comprising two essentially continuous non-porous cationicpermselective diaphragms and spaced from each other to form a centralcompartment of said unit structure, said central compartment having aninlet and an outlet which are an external communication with each otherthrough a conduit, and at least the diaphragm facing the cathodecornpartment comprising (a) a matrix which is polymeric material fromthe class consisting of polyethylene, polyisobutylene, vulcanizednatural and synthetic rubber, polyvinyl chloride and copolymers of vinylchloride and vinyl esters of lower aliphatic acids and (b) particles ofan insoluble infusible cation exchange resin intimately and uniformlydispersed throughout said matrix, said particles being of such size asto pass through a United States Standard sieve No. 50 and being presentin said ilm in an amount equal to twenty-tive percent to seventy-fivepercent of the total weight of said diaphragm, the improvement whichcomprises: introducing the said alkali metal chloride solution into theanode compartment, effecting the electrolysis of said solution byimpressing a decomposition voltage across the electrodes of said cell,whereby hydroxyl ions migrate into said central compartment from saidcathode compartment; introducing into said external conduit carbondioxide to react with said hydroxyl ions; and separately recovering theproducts of the cell so produced.

8. The method of effecting the electrolytic decomposition of an aqueoussolution of an inorganic salt comprising introducing the said solutioninto the anode compartment of an electrolytic cell means having an anodecompartment containing anode means separated from a cathode compartmentcontaining cathode means by a diaphragm unit structure means comprisingtwo essentially continuous non-porous permselective diaphragm meanshaving the same kind of permselectivity and spaced from each other toform a central compartment of said unit structure means, said centralcompartment having an inlet means and an outlet means which are inexternal communication with each other through conduit means, and atleast one of said diaphragms being formed by combining a supportingmaterial with an ion-exchange resin dispersed therein and polymerized toan insoluble infusible condition and sealed to substantialimpermeability to passage of liquids and gases and ions of a given signwhile passing ions of the opposite sign and being composed of materialhaving the characteristics of an ionized salt pair; effecting theelectrolysis of said solution by impressing a decomposition voltageacross the electrodes of said cell, whereby a product of electrolysismigrates into said central compartment; introducing into said externalconduit means a chemical compound reactive with said product ofelectrolysis to form an additional product which under the conditions ofelectrolysis has a diffusion coeicient through the diaphragm no greaterthan about the diffusion coeflicient of carbonate ion through thediaphragm; and separately recovering the products so produced.

9. The method of effecting the electrolytic decomposition of an aqueoussolution of an alkali metal chloride to hydrogen, chlorine, alkali metalhydroxide and an additional product comprising introducing the saidsolution into the anode compartment of an electrolytic cell means,having an anode compartment containing anode means separated from acathode compartment containing cathode means by a diaphragm unitstructure means comprising two essentially continuous non-porouscationic permselective diaphragm means and spaced from each other toform a central compartment of said unit structure means, said centralcompartment having an inlet means and an outlet means which are anexternal communication with each other through conduit means, and atleast the diaphragm means facing the cathode compartment being formed bycombining a supporting material with an ion-exchange resin dispersedtherein and polymerized to an insoluble infusible condition and sealedto substantial impermeability of passage of liquids and gases and anionswhile passing cations and being composed of an ionized salt pair,effecting the electrolysis of said solution by impressing adecomposition voltage across the electrodes of said cell, wherebyhydroxyl ions migrate into said central compartment from said cathodecompartment; introducing into said external conduit carbon dioxide toreact with said hydroxyl ions to form an additional product; andseparately recovering the products so produced.

10. The method of effecting the electrolytic decomposition of an aqueoussolution of an alkali metal salt to alkali metal hydroxide and anadditional product comprising introducing the said salt and water intothe anode compartment of an electrolytic cell means having an anodecompartment containing anode means separated from a cathode compartmentcontaining cathode means by a diaphragm unit structure means comprisingtwo essentially continuous non-porous cationic permselective diaphragmmeans and spaced from each other to form a central compartment of saidunit structure means, said central compartment having an inlet means andan outlet means which are an external communication with each otherthrough conduit means, and at least the diaphragm means facing thecathode compartment being formed by combining a supporting material withan ion-exchange resin dispersed therein and polymerized to an insolubleinfusion condition and sealed to substantial impermeability to passageof liquids and gases and anions while passing cations and being composedof material having the characteristics of an ionized salt pair,effecting the electrolysis of said solution by impressing adecomposition voltage across the electrodes of said cell, wherebyhydroxyl ions migrate into said central compartment from said cathodecompartment; introducing into said external conduit a chemical compoundreactive with said hydroxyl ions to form an additional product whichunder the conditions of electrolysis has a diffusion coeflicient throughthe diaphragm no greater than about the diifusion coeflicient ofcarbonate ion through the diaphragm; and separately recovering theproducts so produced.

11. The process of claim 1 wherein the said reactive chemical compoundis H28.

12. The process of claim 1 wherein the said reactive chemical compoundis H3PO4.

13. The process of claim 1 wherein the said reactive chemical compoundis C12.

References Cited by the Examiner UNITED STATES PATENTS 831,474 9/ 1906Roberts 204-295 1,972,561 9/1934 Henbaum 204-180 2,057,232 lO/l936Endell 204- 2,383,674 8/1941 Osborne 204-87 2,571,247 10/ 1951 Hoebotter204-180 (Other references on following page) UNITED 13 14 STATES PATENTSFOREIGN PATENTS Bodamer 204-180 6,417 1887 Great Britain. Bodamer204-180 Rosenberg 204-108 OTIER REFERENCES Van Hoek 204 1g0 5 Kalauch:Kollold Zeltschrlft, vol. 112 (1949), pp. De Haas Van Dorsser et al.21-26- 04-180 Bodamer 2204 72 JOHN H. MACK, Primary Examiner. Osborne etal. 204-72 JOHN R. SPECK, JOSEPH REBOL'D, MURRAY A.

Osborne et al 204-98 10 TILLMAN, Examiners.

1. IN THE PROCESS FOR THE ELECTROLYTIC DECOMPOSITION OF AN AQUEOUSSOLUTION OF AN INORGANIC SALT IN AN ELECTROLYTIC CELL HAVING AN ANODECOMPARTMENT CONTAINING ANODE MEANS SEPARATED FROM A CATHODE COMPARTMENTCONTAINING CATHODE MEANS BY A DIAPHRAGM UNIT STRUCTURE COMPRISING TWOESSENTIALLY CONTINUOUS NON-POROUS PERMSELECTIVE DIAPHRAGMS HAVING THESAME KIND OF PERMSELECTIVITY AND SPACED FROM EACH OTHER TO FORM ACENTRAL COMPARTMENT OF SAID UNIT STRUCTURE, SAID CENTRAL COMPARTMENTHAVING AN INLET AND AN OUTLET WHICH ARE IN EXTERNAL COMMUNICATION WITHEACH OTHER THROUGH A CONDUIT, AND AT LEAST ONE OF SAID DIAPHRAGMSCOMPRISING (A) A MATRIX WHICH IS POLYMERIC MATERIAL FROM THE CLASSCONSISTING OF POLYETHYLENE, POLYISOBUTYLENE, VULCANIZED NATURAL ANDSYNTHETIC RUBBER, POLYVINYL CHLORIDE AND COPOLYMERS OF VINYL CHLORIDEAND VINYL ESTERS OF LOWER ALIPHATIC ACIDS AND (B) PARTICLES OF ANINSOLUBLE INFUSIBLE ION-EXCHANGE RESIN INTIMATELY AND UNIFORMLYDISPERSED THROUGHOUT SAID MATRIX, SAID PARTICLES BEING OF SUCH SIZE ASTO PASS THROUGH A UNITED STATES STANDARD SIEVE SIZE NO 50 AND BEINGPRESENT IN SAID DIAPHRAGM IN AN AMOUNT EQUAL TO TWENTY-FIVE PERCENT TOSEVENTY-FIVE PERCENT OF THE TOTAL WEIGHT OF SAID DIAPHRAGM, THEIMPROVEMENT WHICH COMPRISES: INTRODUCING THE SAID SALT SOLUTION INTO ANELECTRODE COMPARTMENT; EFFECTING THE ELECTROLYSIS OF SAID SOLUTION BYIMPRESSING A DECOMPOSITION VOLTAGE ACROSS THE ELECTRODES OF SAID CELL,WHEREBY A PRODUCT OF ELECTROLYSIS MIGRATES INTO SAID CENTRALCOMPARTMENT; INTRODUCING INTO SAID EXTERNAL CONDUIT A CHEMICAL COMPOUNDREACTIVE WITH SAID PRODUCT OF ELECTROLYSIS TO FORM A REACTION PRODUCTWHICH UNDER THE CONDITIONS OF ELECTROLYSIS HAS A DIFFUSION COEFFICIENTTHROUGH THE DIAPHRAGM NO GREATER THAN ABOUT THE DIFFUSION COEFFICIENT OFCARBONATE ION THROUGH THE DIAPHRAGM; AND SEPARATELY RECOVERING THEPRODUCTS OF THE CELL SO PRODUCED.